Means for d.c. biasing a device for operation at high frequency



March 24, 1970 E. L. lvr-:Y 3,503,000

MEANS FOR D.C4. BIASING A DEVICE FOR OPERATION AT HIGH FREQUENCY FiledMay 5, 1966 INVENTOR EDWARD L. IVEY ATT RNEY United States Patent O3,503,000 MEANS FOR D.C. BIASING A DEVICE FOR OPERATIN AT HIGH FREQUENCYEdward L. Ivey, Garland, Tex., assignor to Texas InstrumentsIncorporated, Dallas, Tex., a corporation of Delaware Filed May 5, 1966,Ser. No. 547,806 Int. Cl. H03i` 3/14 U.S. Cl. 330--38 5 Claims ABSTRACTF THE DISCLOSURE Disclosed is a microwave circuit having two sections,one having an input for A C. signals and one having a conductor for D.C.bias voltage. The conductor is A.C. coupled to the A C. signal sectionfrom a point of low impedance to the point where the bias A.C. signalsare applied to a device.

This invention relates generally to high frequency circuits, and moreparticularly, but not by way of limitation, relates to method andapparatus for biasing an active component, such as a transistor, with aD.C. voltage or current Without interfering with a microwave signalassociated with the component.

In high frequency circuits, it often becomes necessary to divide an A C.circuit into two or more D.C. isolated sections so that a D.C. bias canbe applied to one of the sections. The conventional method for achievingthis is t0 capacitively couple the two sections to complete the A.C.path while thus maintaining D.C. isolation. Then the D.C. bias isapplied to one of the sections by a meandering line or one-quarterwave-length choke in order to minimize the effect of the D.C. connectionon the A.C. characteristics of the circuit. Several problems areassociated with this approach. The coupling capacitor, due to its size,often produces an impedance discontinuity at microwave frequencies.Further, the D.C. meander line or choke is often quite lossy atmicrowave frequencies.

An important object of this invention is to provide a means for applyinga D.C. voltage or current bias to an A.C. circuit without adverselyinterfering with the A C. characteristics of the circuit.

Another object is to provide such a means that is particularly suitablefor use in thin film microwave circuits.

Another object is to provide means for introducing a D.C. bias to amicrowave circuit which will not create impedance discontinuities andwhich produces virtually no energy loss.

These and other objects are accomplished by A C. coupling a firstconductor for the D.C. bias to a second conductor carrying the A.C.signal from a point of low impedance on the second conductor to thepoint where the bias is to be applied to the circuit. By tightlycoupling the rst conductor to the second conductor between the lowimpedance point and the point where the bias is to be injected, the D.C.bias is actually introduced to the A.C. system at a low impedance pointand therefore does not affect the impedance cha-racteristics of the A.C.signal path. As a result of the close coupling between the twoconductors, the two conductors appear as a single conductor at highfrequency so that energy losses are virtually eliminated, yet the twoconductors are D.C. isolated.

In accordance with a more specific aspect of the invention, a highfrequency thin film circuit is provided wherein a strip transmissionline conductor carrying an A.CA signal is terminated at ground through atuning stub or the like. The D.C. bias conductor is closely coupled to,but is D.C. isolated from, the A.C. signal strip line conductor from apoint adjacent the ground potential to the point on 3,503,000 PatentedMar. 24, 1970 the A C. conductor Where the A.C. signal is to be coupledto the section of the circuit to be biased. The D.C. conductor is thenD.C. coupled to the section of the circuit to 'be D.C. biased. Inaccordance with a more specific aspect of the invention, an `activecomponent, such as a transistor, may be biased using this techniquewherein one terminal, such as the collector, is grounded, and a D.C.bias is applied to one or more of the other terminals using the methodjust described.

The novel features believed characteristic of this invention are setforth in the appended claims. The invention itself, however, as Well asother objects and advantages thereof, may best be understood byreference to the following detailed description of an illustrativeembodiment, when read in conjunction with the accompanying drawings,wherein:

FIGURE 1 is a plan view of a thin film microwave circuit constructed inaccordance with the present invention;

FIGURE 2 is an enlarged view of the central section of the circuitillustrated in FIGURE l;

FIGURE 3 is a sectional view taken substantially on lines 3-3 of FIGURE1;

FIGURE 4 is a sectional view taken substantially on lines 4--4 of FIGURE1;

FIGURE 5 is a sectional view taken substantially on lines 5-5 of FIGURE1; and

FIGURE 6 is a schematic equivalent circuit diagram of the circuitillustrated in FIGURE 1.

Referring now to the drawing, a circuit constructed in accordance withthe present invention is indicated generally by the reference numeral10. The circuit 10 is a microwave amplifier stage comprised of atransistor 12 and input and output tuning means to achieve input andoutput characteristic impedances of 50 ohms. The tuning circuitryassociated with the transistor 12 is fabricated using known thin filmtechniques on a ceramic substrate 14 having a uniform thickness. Ametallized film 16 is deposited on the underside of the substrate 14 andforms a ground plane so that all strip conductors form striptransmission lines. An input conductor 18 is, therefore, a striptransmission line by reason of its association with the ground plane 16.The input transmission line 18 is connected to the input 20 of aquarterwave transformer section 22 which may be considered asterminating generally at an output point 24. However, the stripconductor continues through a tuning stub section 26 to a ground strap28 which is electrically connected to the ground plane 16 by means notillustrated and which is, therefore, at ground potential.

A D.C. biasing conductor 30 extends from an expanded contact pad 32 toan enlarged capacitor plate section 34 and finally a bonding tab 36 asseen in FIGURE 3. The conductor 30 is electrically insulated from theconductor 26 by a thin insulating layer 38. The expanded contact pad 32is preferably formed directly on the substrate 14 within an opening 40formed in the ground strap 28. It will be noted that the insulatinglayer 38 extends partially into the opening 40 to prevent the conductor30 from contacting the edge of the ground strap 28. Similarly, theinsulating layer 38 has an expanded portion 42 which extends over theedges of the strip conductor forming the transformer 22 and tuning stub26 so that the contact tab 36 may extend onto the ceramic substrate 14as best seen in FIGURE 3. Placing the bonding tab 36 and the expandedcontact 32 directly on the ceramic substrate mechanically facilitatessubsequent electrical contact with these metal films without damage tothe circuit. The insulating layer 38 may be a very thi-n layer of asuitable insulating oxide deposited using the standard thin filmtechniques so as to have maximum A C. coupling between the line 26 andline 30 while still maintaining D.C. isolation for purposes which willpresently be described.

An expanded collector contact pad 44 is formed on the ceramic substrate14 and is connected through a meandering tuning line 46 to an outputstrip line conductor 48. The meandering tuning line continues through asection 50 and terminates at a ground strap 52 which is integral withthe ground strap 28. A second D.C. biasing conductor 54 extends from anexpanded contact pad 56 formed on the substrate in an opening in theground strap 52 to a bonding tab 58, and is D.C. isolated from theunderlying ground strap 52 by a layer of insulating material 59, whichmay also be a suitable conventional oxide film. The conductor 54 has anenlarged portion 60 for increasing the efficiency of the A.C. couplingbetween the conductor 54 and the ground strap 52. The transistor 12 ismounted directly upon the expanded pad 44 so that the collector of thetransistor will be in good electrical contact with the pad. A bondedlead wire 62 interconnects the base contact of the transistor andbonding tab 36, and a bonded lead wire 64 interconnects the emittercontact of the transistor and the bonding tab 58. A D.C. bias voltagemay be applied to pads 32 and 56 by any suitable means such as by asolder or pressure connection.

The biasing circuit illustrated in FIGURE 1 may be roughly representedby the equivalent circuit illustrated in FIGURE 6 wherein correspondingparts are represented by corresponding reference numerals. The striptransmission line sections 22 and 26 may be represented by coils. Thestrip conductor 30, including the expanded portion 34, may berepresented collectively by a coil, it being understood that the twocoils 26 and 30 are very tightly coupled inductively by reason of theirclose proximity. Also, the strip conductor 30 is capacitively coupled tothe conductor 26 throughout its entire length, and the capacitivecoupling is enhanced by the expanded portion 34. This capacitivecoupling is lumped and represented by capacitor 30-34 in FIGURE 6. It isimportant to note that although the conductor 30 has an expanded portion34, the capacitive coupling does occur for the entire length of theconductor 30 and that the enlarged portion 34 is provided merely toincrease the coupling eticiency. If desired, the conductor 30 may alsoextend along the conductor 22 to enhance the A.C. coupling. Capacitors70 and 72 represent the capacitive coupling between the conductors 26and 30 and the ground plane 16, respectively, although this capacitanceis actually present over the length of the respective conductor. D.C.bias for the base of transistor 1`2 is applied to the terminal pad 32.

At microwave frequencies, the conductor 30 and the conductor 26 areequivalent to a single conductor as a result of the very tight A.C.coupling between the two conductors. Since the D.C. Ibias is applied toexpanded contact 32 at a point of essentially zero impedance in themicrowave circuit, the application of the D.C. bias does not disturb themicrowave characteristics of the circuit. The function of thetransformer 22 and the tuning stub 26 is well known and does not, perse, constitute a part of the present invention. The transformer 22 isused to establish a characteristic input impedance of ohms in theinstant case, and the tuning stub 26 is used to cancel the imaginarycomponents of the microwave energy. Since the D.C. bias applied toexpanded contact terminal 32 is D.C. isolated from the strip conductors26 and 22, the base of transistor 12 may be D.C. biased so that thetransistor 12 will be operated under optimum conditions to amplify themicrowave signal applied through input 18 and A.C. coupled to the baseof the transistor 12 through the D.C. bias line 30. Since the collectorof the transistor is already at ground, the meandering striptransmission tuning line 46 is provided merely to establish the desired50 ohm characteristic output impedance, and the meandering tuning stub50 is provided to cancel out the imaginary components of the microwaveenergy, The emitter of the transistor 12 is biased by the potentialapplied to expanded contact terminal 56. The capacitive coupling betweenthe D.C. bias conductor 54, as enhanced by the expanded portion 60, andthe underlying ground strap 52 provides an emitter bypass which shortcircuits microwave energy to ground.

Thus, it will be noted that in accordance with the method of the presentinvention, a D.C. bias is introduced at a low impedance point in an A.C.circuit path, preferably a point of essentially zero impedance, asrepresented by a point at essentially ground potential. A conductorcarrying the D.C. bias potential is then extended along the A.C. circuitpath from the low impedance point to the point at which an a D.C. signalis to be transferred between D.C. isolated sections of the A.C. circuitand the conductor is tightly coupled, capacitively and inductively, tothe A.C. circuit over substantially its entire length.

Although in the particular embodiment selected to illustrate theinvention the D.C. bias is applied to the base of a transistor 12, thesame technique may be used to `bias the emitter, or the collector of atransistor, or to bias any other component or portion of the circuitdesired.

Although a preferred embodiment of the invention has been described indetail, it is to be understood that various changes, substitutions andalterations can be made therein without departing from the spirit andscope of the invention.

What is claimed is:

1. In a microwave circuit comprising in combination:

(a) a semiconductor device;

(b) a microwave transmission line for coupling a microwave signal to theinput of said device, said line includes a tuning stub that terminatesremote from the input of said device;

(c) a D.C. biasing conductor for connecting a D.C.

voltage to the input of said device; wherein (d) said D.C. biasingconductor overlies but is insulated from said tuning stub of said lineso as to be D.C. isolated from said line but A.C. coupled to said linethrough the distributed capacitance between said tuning stub of saidline and substantially the entire length of said conductor;

(e) a D.C. voltage connected to said conductor; and

(f) an A.C. signal coupled to the other end of said line; wherein (g)said D.C. voltage does not interfere with the A.C.

characteristics of said circuit, and said line and conductor operate asa single conductor at microwave frequencies so as to substantiallyminimize energy losses in said circuit.

2. The microwave circuit of claim 1 wherein (a) said microwavetransmission line is a thin conductive film formed on an insulatingsubstrate; and wherein (b) said D.C. biasing conductor is a thinconductive lm overlying said line; and wherein (c) said conductor isinsulated from said line by a thin layer of insulating material.

3. The microwave circuit of claim 1 wherein said semiconductor device isa transistor having one of its electrodes D.C. connected to said D.C.biasing conductor and A.C. coupled to said microwave transmission line.

4. In a microwave thin film circuit comprising in combination:

(a) a semiconductor device mounted on an insulating substrate;

(b) a thin film microwave transmission line formed on said substrate,said line including a transformer section connected to a tuning stubthat terminate at a reference potential with the junction of saidsection and stub being A.C. coupled to the input of said device forcoupling a microwave signal thereto; and

(c) a thin film D.C. biasing conductor ymounted on said substrate inoverlapping relationship to but insulated from at least Said .Stub ofsaid line, said conductor having one end D.C. connected to the input ofsaid device for connecting a D.C. voltage thereto; wherein (d)4 saidconductor is D.C. isolated from said line but A C. coupled to said linethrough the distributed capacitance between at least said stub of saidline and substantially the entire length of said conductor; and wherein(e) said D.C. voltage is connected to the unconnected end of saidconductor so as to connect said D.C. voltage to the input of said deviceat a relatively low impedance point of the circuit; and wherein (f) saidA C. signal is coupled to the unconnected end of said line so as tocouple said signal to the input of said device from said junction ofsaid section and stub; whereby (g) said D.C. voltage does not interferewith the` A.C.

characteristics of said circuit, and said line and conductor operate asa single conductor at microwave frequencies so as to substantiallyminimize energy losses in said circuit.

5. In a microwave circuit comprising in combination:

(a) a semiconductor device;

(b) a microwave transmission line for coupling a microwave signal to theinput of said device said line includes a quarterwave transformersection with one of its ends connected to a quarterwave tuning stub thatterminates to ground; and

(c) a D.C. biasing conductor for connecting a D.C. voltage to the inputof said device said D.C. biasing conductor substantially overlies but isinsulated from the entire length of said tuning stub with one end D.C.connected to the input of said device so as to be D.C. isolated fromsaid line but A.C. coupled to said line through through the distributedcapacitance .between said line and substantially the entire length ofsaid conductor; wherein (d) said D.C. voltage is connected to theunconnected end of said conductor so as to connect said D.C. voltage tosaid circuit at relatively low impedance point; and wherein (e) saidA.C. signal is coupled to the unconnected end of said quarterwavesection; and wherein (f) said A.C. signal is injected into said input ofsaid device from the junction of said transformer section and tuningstub; whereby (g) said D.C. voltage does not interfere with the A.C.

characteristics of said circuit, and said line and conductor operate asa single conductor at microwave frequencies so as to substantiallyminimize energy losses in said circuit.

References Cited UNITED STATES PATENTS 2,926,308 2/1960 Thanos 330-403,189,823 6/ 1965 Mitchell. 3,246,256 4/ 1966 Sommers 331-107 3,155,88111/1964 St. Jean.

ROY LAKE, Primary Examiner J. B. MULLINS, Assistant Examiner U.S. C1.X.R.

